When Barack and Michelle Obamawere married in Chicago two decades ago, Santita Jackson, a daughter of the Rev.Jesse L. Jackson, sang at their wedding. When Mr. Obama ran for his first national office, he made sure he was not stepping on the ambitions of her brother,Jesse L. Jackson Jr., who later became a co-chairman of his 2008 presidential campaign.

Now the younger Mr. Jackson, 47, who served 17 years as a congressman representing his hometown, is most likely headed to prison for campaign fraud, trailed by a string of problems from an extramarital affair to mental illness. Although the fates of Mr. Jackson and Mr. Obama could not be more different, their stories, and those of their families, are bound together. The rise of the current leading black political family in the United States is inextricable from the unraveling of an older one, with the two tangled in shifting alliances, sudden reversals of fortune and splits.

Decades ago in Chicago, Mr. Jackson was seen as a far more promising figure than his friend Mr. Obama — one the heir to a legend, the other an outsider seeking to surpass the father he barely knew. If Mr. Jackson had decided to run for the United States Senate in 2004, Mr. Obama most likely would not be president. That year and again in 2008, Mr. Obama, seeking to bolster his credibility with African-Americans, enlisted the younger Mr. Jackson for crucial help.

But along the way, the Jackson father and son helped define what the future president did not want to become: a black politician mired in the old urban-ethnic mold; a leader tainted by personal transgressions or a dysfunctional family. Mr. Obama, who saw the Jacksons’ problems up close, increasingly kept his distance from both men. Now, the Obama administration’s Department of Justice is poised to send the younger Mr. Jackson — who came under investigation after he pursued but failed to obtain Mr. Obama’s old Senate seat — to prison.

“The beginning of Jackson’s star being tarnished was the beginning of Obama’s star rising,” Eric Adelstein, a Chicago political consultant, said in an interview. “You can plot it on a chart: as Obama ascends, Jackson descends.”

Decades before Mrs. Obama worked on her husband’s presidential campaigns, she saw the process through the races of the elder Mr. Jackson. Her father, Fraser Robinson, was a city water worker and precinct captain, but Ms. Jackson, the oldest daughter of the most important black politician in the country, was one of her closest friends.

“Michelle used to come to the house all the time,” Mr. Jackson recalled in an interview. (The White House declined to comment for this article.)

The man she eventually married was a stranger to Chicago, who had to pound the sidewalks of the South Side to collect signatures to run for a lowly State Senate seat in 1996. Mr. Obama would regularly attend weekly meetings of Mr. Jackson’s Rainbow/PUSH organization, and the Jacksons tried to be helpful with advice and opportunities — for example, giving him chances to “get his speaking legs,” said Hermene D. Hartman, publisher of an African-American newspaper, N’DIGO.

“I don’t think the Jacksons were threatened particularly,” said Don Rose, a longtime political consultant.

They had no reason to be. Jesse Jackson Jr. was already a sensation: at just 30, he had defeated more established politicians for a Congressional seat. His father proudly raised money and campaigned on his behalf, at one point distributing buttons that read “A new generation.” Some in Chicago saw strengths in Mr. Jackson that they had not seen in his father — crossover appeal to white voters, expertise in both politics and policy, an ability to move between the civil rights movement and elite institutions like St. Albans, his prestigious Washington school. Soon, People magazine named him sexiest politician.

“If I wanted to be an elected official, that’s not enough,” Mr. Jackson told The Chicago Tribune in 1995, speaking of the expectations he faced. The jobs he talked about pursuing were mayor of Chicago or speaker of the House, but his father and others went even further, Mr. Jackson noted: “One day, son, you may be president.”

It is not clear why he did not run in 2004 for a Senate seat that some thought could be his and gave Mr. Obama the go-ahead instead. Some say Mr. Jackson had set his sights on other offices; others say he did not think Mr. Obama, who had been trounced in a 2000 Congressional race, had a chance of winning.

He gave Mr. Obama a much-needed endorsement in the Democratic primary. “Jesse had him on 100 billboards,” said the elder Mr. Jackson, when Mr. Obama’s name “was hardly known.” Santita Jackson was godmother to young Malia Obama.

Still, Mr. Obama was wary of being identified with the older generation of Chicago politics, particularly the elder Mr. Jackson, whose fiery civil rights rhetoric often alienated whites and whose reputation was sullied by a messy personal life, including fathering a child out of wedlock. Campaign aides wanted the Obamas alone on stage the night he won the primary, two of them said later, without other local politicians — especially the elder Mr. Jackson, who had a habit of making a beeline for the victor at the moment cameras clicked.

A few months later, when the still-obscure candidate became the toast of the Democratic Party after a soaring speech at the presidential nominating convention for SenatorJohn Kerry, several people noticed that both Jackson men seemed disoriented by the turn of events.

“They were the biggest thing in African-American politics,” said Jim Cauley, Mr. Obama’s campaign manager in the Senate race, and in one night, “they got eclipsed.”

The younger Mr. Jackson felt that Mr. Obama “was sort of elevated to his status without having to pay his dues or do grunt work,” saidAlexi Giannoulias, a former state treasurer. “Which was simply untrue — he worked incredibly hard as a candidate and he did so all across the state, which is something Junior never had to do. But I think in his mind, somewhere deep down, Junior felt that way.”

After that, the younger Mr. Jackson appeared to grow even more determined. He transformed himself with weight loss surgery and considered a run against MayorRichard M. Daleyin 2007.

In the 2008 presidential race, both Jackson men endorsed Mr. Obama. But though the congressman served as a co-chairman of his campaign, his father openly seethed at Mr. Obama, criticizing him at turns for “acting like he’s white,” not appearing at a civil rights anniversary and “talking down to black people.” He made a crass anatomical remark about Mr. Obama, unaware that he was speaking on a live microphone. His son issued a rebuke. “He should keep hope alive,” he said, “and any personal attacks and insults to himself.”

The elder Mr. Jackson’s influence was already on the wane, but the hostility he showed toward Mr. Obama, who was winning the devotion of black voters, sealed his loss of prestige. Mr. Obama’s attitude toward the older man was “I’m bigger than this, so I’m just going to ignore you,” said one fund-raiser. As the race progressed, the outbursts from Mr. Jackson drew the candidate closer to the Rev.Al Sharpton, the civil rights activist.

“He ended up fighting the people he helped make way for,” Mr. Sharpton said of the senior Mr. Jackson.

Mr. Obama’s presidential election victory seemed to create the opportunity the younger Mr. Jackson had been seeking. He lobbied hard for Mr. Obama’s Senate seat, petitioning Gov.Rod R. Blagojevich, who would make the appointment; seeking letters of support from opinion makers; publicizing a poll his camp had commissioned.

Instead, his life fell apart. Mr. Blagojevich was arrested, charged with trying to solicit money or a job in exchange for the seat, and a friend of Mr. Jackson’s was accused of offering large campaign contributions if Mr. Jackson was picked. Mr. Jackson denied knowing about the offer, but a House ethics committee investigation was opened.

In 2010, revelations emerged of an extramarital relationship. By 2011, few even seemed to think of him as a contender for the job of mayor of Chicago after Mr. Daley announced plans to retire. And by June 2012, he disappeared from Congress, his office eventually announcing that he was being treated for bipolar disorder. He resigned in November, and pleaded guilty last week to a felony fraud count for spending campaign funds on living expenses and collectibles.

Through a spokesman, Mr. Jackson declined to be interviewed for this article, but issued a statement apologizing to those he let down.

Since becoming president, Mr. Obama has had dwindling contact with the Jacksons. The son was under investigation and the father was persona non grata, absent from civil rights meetings Mr. Obama has held, according to participants, despite the role Mr. Jackson played in the movement and in helping to clear the way for a black man to become president.

“By the time he came along, he was going across a bridge that came from the rocks of the walls that we knocked down,” Mr. Jackson said in the interview.

He had just returned from Washington, where he appeared in court with his son, and he sounded weary. But he discussed the president with none of his old acrimony. Mr. Obama, he said, was a “rare genius.”

He seemed to savor one rare moment he has shared with Mr. Obama in recent years, a quick staged photograph after the dedication of the memorial to the Rev. Dr.Martin Luther King Jr.in 2011. The Jacksons waited with other civil rights leaders for their shot with the Obamas, but Mr. Jackson described the encounter as a reunion for the two families, who spent a few minutes exchanging pleasantries and memories.

He said he spends little time worrying about his standing at the White House or invitations there, adding, “I give him room to govern.”

Correction: March 5, 2013

An article on Feb. 25 about the relationship between the families of President Obama and the Rev. Jesse L. Jackson misidentified the political post once held by Fraser Robinson, Michelle Obama’s father. He served as a precinct captain for Chicago Democrats, not as a ward captain.

A version of this article appears in print on February 25, 2013, on page A1 of the New York edition with the headline: Crossed Paths: Chicago’s Jacksons and Obamas. Order Reprints|Today's Paper|Subscribe